The positioning of a raster scan line, like that generated by the optics of a polygon laser printer such as shown in U.S. patent application Ser. No. 07/637,429, filed Jan. 4, 1991, onto an x-ray film or other photosensitive image recording medium involves assuring accurate alignment in the x (cross-scan), y (scan) and z (beam path) translational directions, as well as in the .theta..sub.x, .theta..sub.y and .theta..sub.z rotational directions respectively about each of the x, y and z directions. Proper positioning also requires attainment of the right angle of incidence of the beam onto the target medium.
Excessive focal shift errors of the scan line with respect to the recording medium can lead to a printing artifact called banding. Banding is an undesirable periodic density modulation in the image that is most noticeable in uniform density regions. A very high degree of accuracy of scan line placement is required especially for laser printers used for x-ray films, because banding artifacts can lead to false diagnoses. The eye is very sensitive to low frequency (1-8 cycles/degree) contrast modulation, having a threshold as low as 0.2% under the right illumination conditions. The design goal typically will use half this tolerance, so it will have 0.1% allowable line shift for the extreme polygon facet error. For a printer with a line pitch of 0.08 mm, the allowable error is thus only 80.times.10.sup.-9 mm. This requirement means that only a very small focus error can be present for the page cross section of the beam.
Antos et al. U.S. Pat. No. 4,397,521 discloses a polygon raster scanner having a pair of adjustable screws which cooperate with a leaf spring to precisely aim a laser beam toward the optical system. Jeffery et al. U.S. Pat. No. 4,043,632 discloses a polygon mirror with adjustable facets. Kramer U. S. Pat. No. 4,826,268 discusses banding in a hologon scanner. Oosaka et al. U. S. Pat. No. 3,922,060 discloses the use of slidable "v" pads for accurate motion of a movable stage for an optical bench in hologram recorder. Other patents describing related subject matter include the following U.S. Pat. Nos.: 4,040,097; 4,239,326; 4,243,293; 4,304,459; 4,487,472; and 4,904,034.
Scan line generating optics systems are susceptible to two types of focusing errors: those which cause deviations of the scan curve from a straight line; and those which displace the scan line from its intended generatrix line on the target drum. The f-.theta. condition correcting and other optical elements can be manufactured with sufficiently tight tolerances and alignment procedures to ensure the generation of an acceptably straight (i.e. non-arcuate) scan line in space. This invention addresses a method and apparatus for ensuring proper placement of the spatially generated scan line onto the recording medium.